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nation, as well as the varying character of stones in different parts of 
a quarry, and it was thought that inattention to these circumstances 
was one great cause of the rapid decay of many modern buildings. He 
then proceeded to detail the several causes of decay, and said that 
practically the destruction of absorbent stones is connected with 
exposure to a damp atmosphere, rendered impure by various acid and 
alkaline vapours, and also with changes of temperature, especially 
above and below 38°, at which water obtains its greatest density. The 
deleterious substances are oxygen, carbonic acid, nitric acid, ammonia, 
and water of the air, and to these, in towns, may be added sulphurous, 
sulphuric, and hydrochloric acids. Carbonic acid in solution, as in rain, 
is a powerful solvent of all kinds of calcareous matter; in calcareous 
and magnesian rocks it acts by transforming the insoluble into soluble 
carbonates, thus removing the lime and magnesia from the stone, for 
although the quantity is small the action of the solvent is continuous. 
On other rocks it assists disruption, by the solution of the material 
cementing the particles together, and eventually making all yield to 
its influence. The various acids mentioned must be looked upon as 
amongst the principal agents in effecting the destruction of stone. They 
result from combustion and various manufacturing processes carried 
on in towns. Dr. A. Smith has shown that the air of the central part 
of Manchester contains twenty-five parts of sulphuric acid in 100,000 
of air. This acid acts directly on the magnesian limestones, and 
indirectly on sandstones, by the solution of the cementing materials, 
and thus, besides destruction, causes increased porosity of stone, 
rendering it more amenable to the action of water and frost. 
Besides this, in magnesian stones there is the formation of sulphate 
of magnesia, remarkable for the large amount of water of crystalliza¬ 
tion which it contains; the powerful mechanical effects resulting from 
the solidification of this water produces effects similar to those pro¬ 
duced by the expansion of freezing water, and is the basis of Brard’s 
test for the value of building stones. Nitrification is similar to the 
preceding, and arises from the formation of various nitrates or 
sulphates in the stone, under certain circumstances and in certain 
situations, which were detailed. This change displays itself by 
the formation of minute crystals, efflorescing from the interior to the 
exterior of the stone, and leading ultimately to its disruption. The 
porosity of a stone, either natural or induced by any of the previous 
causes, is important in estimating its stability, for water, when absorbed, 
acts upon stone quite as much, through the changes in its volume at 
c 
